Electrically conductive cements and adhesives are typically fabricated from single- or multi-component non-conductive carrier materials and conductive fillers such as metal or metallic particulate. While various cements can be used as the carrier, multi-component epoxies, single-component solvent-based systems, and combinations thereof have been used. Epoxies have a long shelf life, good bonding properties and can be cured with many materials. Similarly, single-component solvent-based systems can be readily cured by driving off the solvent to form a strong and reliable bond with many materials. The fillers are typically noble metals, such as, gold or silver, in various particulate sizes. A preferred filler is a mixture of flake-like and non-flake particles of various sizes. The particles can be essentially solid or, in some cases, metal-plated non-conductive bodies. In a typical formulation, the conductive filler can comprise approximately 75% by weight, or more, of the total material with the carrier comprising the remaining material. It has been thought that metal or metal-plated particles in the form of flakes or platelets provide the bulk conductivity characteristics of such conductive cements because the flakes tend to align themselves in a continuous overlapping relationship in the cured carrier to provide an electron pathway. Non-flake conductive fillers in the carrier are thought to fill interstices between the flake particles providing enhanced conductivity.
Modern electric circuits may be fabricated as traditional rigid printed circuit boards (PCBs) using a subtractive process in which copper traces and connection pads form circuits which are etched from a copper foil layer attached to a rigid, non-conductive board or substrate. Electrical components are connected to such circuits by passing their usually solder-plated leads through mounting holes in the board and connecting the leads to connection pads by lead/tin soldering. It is also known to fabricate so-called `flex circuits` in which copper traces and connection pads are formed on a flexible layer or substrate of, e.g., polyimide or polyester sheet, such as KAPTON.TM. polyimide in the 1 to 5 mil thickness range. Electronic components are connected to such circuits by passing their leads through mounting holes in the flexible layer and connecting the leads to connection pads by lead/tin soldering. More recently, surface-mount components (SMC) have been developed in which the component leads are merely positioned upon connection pads and soldered into place forming a butt joint. Surface-mount technology can be used with both rigid and flexible substrates.
Traditional soldered-connection systems represent highly developed technologies with proven performance under various temperature and humidity conditions, particularly higher-temperature and higher-humidity conditions. However, traditional soldered-connection systems often involve extensive chemical processing with various types of etchants and similar chemicals to fabricate a circuit on a substrate and may also require various fluxes and solvents to effect soldered connections. Additionally, traditional soldered-connection systems involve the application of a substantial amount of heat to momentarily melt the solder material to effect the connection. While rigid substrates and certain relatively high-cost polyimides are designed to accommodate the heat of soldering, flexible substrates that use the lower cost polymers such as polyesters are more susceptible to heat damage because of their relatively thin cross-section, low-heat capacity, and susceptibility to distortion. For example, soldered connections on a flexible polyester substrate can cause local `puckering` of the substrate, changes in the center-to-center dimensions of the various connection pads, and warpage of the entire substrate.
Efforts have been made to use electrically conductive inks, cements, and adhesives to replace traditional soldered-connection systems in both rigid and flexible substrate applications. For example, electrically conductive ink circuits, including connection pads, have been printed on flexible polyester substrate and the conductive terminals of SMC devices then cemented to the connection pads with electrically conductive cement. Advantageously, the resulting flexible printed circuit can be easily configured to fit into a particular mounting envelope providing enhanced design flexibility. In addition, it is estimated that a successful solderless-connection system utilizing conductive inks, cements, and low-cost flexible substrates could provide significant cost savings over traditional soldered-connection systems. The term `conductive cement` as used herein means any composition or material used to establish electrical contact and a mechanical connection of separate bodies, e.g., a lead and a connection pad.
While conventional electrically conductive cements do not ordinarily possess the conductivity of solid metals and solder alloys, their conductivity (for example, 100 milli-ohm per cemented connection compared to 10 milli-ohm per soldered connection) is adequate for many electrical circuits. For example, a junction resistance of one ohm or so between a component lead and its connection pad will have little effect where the component is a resistor or other device having a resistance or impedance of several hundred or thousand ohms or greater. While junction resistance becomes more important in low-impedance circuit applications, a circuit can usually be designed to accommodate a wide range of cumulative junction resistances. In addition to the quantitive aspect of junction resistance, stability or small changes in junction resistance with time and environment is also an important aspect. A connection system that provides connections having a known resistance that is stable over time and under different environmental conditions is desirable because a connection system that does not provide the requisite stability would be unsuitable for many applications. In the context of a solderless connection system employing electrically conductive cement, any connection which on average exhibits less than about 20 to 25%, and preferably less than about 15% change in junction resistance after 1000 hours of exposure to 90% Relative Humidity (R.H.) at 60.degree. C., is generally considered acceptable. The terms `moisture resistant cement` and `moisture resistant electrical contact` as used herein refer to a conductive cement that provides connections having stable junction resistance that on average does not change more than about 25%, under the test conditions described herein, i.e., after about 1000 hours exposure to 90% R.H., at 60.degree. C.
One factor that affects the electrical conductivity at a junction interface is the presence or absence of non-conductive or resistive surface oxides that form as a consequence of exposure of the surfaces to be joined to ambient air and moisture. In soldered-connection systems, oxides are largely removed at the interfacial boundary between the solder-plated lead and the solder itself by fluxes that react with and effectively remove the oxides and which also serve to shield the junction interface from the ambient atmosphere and moisture as the solder cools. In conductive cement connection systems fluxes are not present during the curing process; and, therefore, it is desirable that the cement include means for reducing the adverse effects of surface oxides without the need to treat the surfaces to be connected, e.g., leaded electrical component leads, with aggressive cleaning agents or other treatments prior to effecting the connection.
While the initial bulk resistivity of known silver-based conductive cements is adequate, such cements are susceptible to increases in their resistivity at the interfacial boundary with a soldered-plated lead due to non-conductive lead/tin oxides. Thus the resistance across the junction between a conductive cement and a soldered-plated lead can vary considerably over time particularly with exposure to high humidity. The resistance of such connections is particularly sensitive to continued exposure to combinations of high humidity and high temperature. Since the above-described single- or double-component polymeric carriers used in conventional conductive cements are inherently permeable to moisture, the connections made with such cements are to some extent subject to the adverse effects of moisture. While a circuit can be designed to accommodate cumulative junction resistance, changes in that resistance with time can have a detrimental effect on the circuit's overall electrical performance. It is believed that moisture which permeates the carrier, in a connection formed with conductive cement, oxidizes metal at the interfacial boundary between the conductive cement and the component lead and the resultant non-conductive oxides tend to increase resistance.
Conductive cement compositions known in the art are typically comprised of polymeric carriers filled with conductive particles. For example, U.S. Pat. No. 4,880,570 describes a mixture of epoxy based adhesive, catalyst and conductive particles shaped to minimize steric interference and provide conductivity; U.S. Pat. No. 4,859,364 describes a mixture of organic medium filled with conductive particles of 0.3 to 1.0 micrometer and conductive metal coated particles of not more than 1 micrometer; U.S. Pat. No. 4,859,268 describes a mixture of photosensitive epoxy polymer, plasticizer and spherical electrically conductive particles; U.S. Pat. No. 4,814,040 describes an adhesive layer including conductive particles sized to penetrate a resistance layer and invade a metallic pattern by a thermal compressions process; U.S. Pat. No. 4,732,702 describes a mixture of resin filled with electroconductive metal powder or an inorganic insulating powder coated with electroconductive film; U.S. Pat. No. 4,716,081 describes a mixture of plastic, rubber or resin filled with silver-surfaced metal particles; U.S. Pat. No. 4,701,279 describes a mixture of thermoplastic elastomer filled with metal particles; U.S. Pat. No. 4,696,764 describes a mixture of resin filled with both abrasive and fine conductive particles; U.S. Pat. No. 4,624,801 describes a mixture of polyester urethane based polymer admixed with a blocked isocyanate and filled with conductive particles; U.S. Pat. No. 4,747,968 describes a mixture of epoxy resin, hardener and metallic silver particles; U.S. Pat. No. 4,564,563 describes a mixture of acrylic, carboxylated vinyl and epoxy polymer filled with metallic silver particles; and U.S. Pat. No. 4,566,990 describes a mixture of thermoplastic condensation polymer filled with both metal flake and conductive metal or metal coated fiber.
In general, conductive cements based on silver-filled polymeric systems perform well over a reasonably large temperature range but do not tend to perform well at the interfacial boundary between the cement and electrical leads under high humidity conditions. When aged under high humidity conditions, the resistance across the interfacial boundary is usually unstable, i.e., increases significantly. While many circuits can operate adequately with such increases in the resistivity in one or more of their connections, the humidity sensitivity is considered a factor limiting more widespread use of conductive cements in both rigid and flexible substrate applications.